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Experimental

1. Dissolve 13.69 g (80.53 mmol) of dihydropyrimidin-2(3H)-one 9 in 500 mL of anhydrous tetrahydrofuran and stir at room temperature under a nitrogen atmosphere.

2. Add lithium bis(trimethylsilyl)amide (161 ml, 161 mmol, 1M in tetrahydrofuran), dropwise, over ~30 min.

3. Portionwise, add 20.19 g (80.53 mmol, recrystallized) of (1S)-(+)-10-camphorsulfonyl chloride and stir for another 30 min.

4. Quench the reaction with saturated ammonium chloride.

5. Partition the reaction in ethyl acetate and water.

6. Dry the organic layers in sodium sulfate and vacuum filter the resulting mixture.

7. Purify the 1:1 mixture of diastereoisomers in flash chromatography using 40% ethyl acetate.

8. Approximately 10.85 g of 10a will be obtained as a colorless oil.

Procedures

· Portionwise addition means that the substance should be added in small fractions at a time instead of the whole reagent at once.

· Partitioning also goes by the name liquid-liquid extraction. It separates solutions based on which type of solvent they dissolve into and their density.

· Drying a liquid is achieved by adding a hydrate salt so that the salt draws out all of the water, leaving the organic solvent. Drying is normally followed by vacuum filtration.

· Vacuum filtration removes large particulates and solids from a liquid by pulling it through a fine filter.

· Flash chromatography uses the different polarities of compounds in solution to draw them through silica gel at different rates. It is a common purification practice in organic chemistry when separating isomers that were formed in solution together.

Chemicals

Dihydropyrimidin-2(3H)-one 9 - starting material

Tetrahydrofuran - solvent

Bis(trimethylsilyl)amide - strong base

(1S)-(+)-10-camphorsulfonyl chloride - chiral reagent and stereocenter marker

Ammonium chloride - aqueous weak acid

Water - aqueous extraction solvent